1. Field of the Invention
The present invention relates generally to abnormality detection devices and more specifically to an abnormality detection device suited for use with automotive vehicle speed sensors and the like.
2. Description of the Prior Art
In order to control automatic transmissions and similar arrangements in automotive vehicles it is known to sense the vehicle speed and produce an electric signal indicative thereof.
In view of the importance of the signal to the control of the vehicle such systems have been provided with abnormal operation (malfunction) detection circuits for detecting a speed sensor malfunction. One example of such a vehicle speed sensor/abnormality detection circuit is disclosed in TOYOTA CROWN OPERATORS MANUAL published on Aug. 31, 1983 by TOYOTA AUTOMOTIVE Co. Ltd SERVICE SECTION PUBLICATION pages 4-26 to 4-27.
The device disclosed in this publication is such that if the vehicle speed sensor malfunctions during the running of the vehicle, the abnormality detection circuit is able to detect the change. However, if the vehicle speed sensor malfunctions from the outset (viz., prior the vehicle being driven) the malfunction cannot be discovered.
In order to overcome this drawback the entity to which the instant application is assigned, proposed an arrangement (disclosed in JM-A-61-9707) wherein more than two vehicle speed sensors were provided in the vehicle. In this arrangement when one of the sensors produced a signal indicative of a predetermined vehicle speed, irrespective of no signals being produced by the other sensors, the sensor in question was indicated as being out of order.
With this arrangement one of the sensors is primarily used and only when this main sensor is detected as being out of order, is the output of the an auxiliary sensor used. However, as the malfunctioning sensor is still connected to the system, it sometimes occurs that due to vehicle vibration or the like that the primary sensor accidentally produces an output pulse. Under these circumstances, even though this sensor has been designated as being out of order, it is re-evaluated as being functional and the output of the sensor is, even though it does not necessarily accurately represent the vehicle speed, is used for the purposes of transmission shift control and the like.
By way of example, as shown in FIG. 5 at time t1 the primary or first speed sensor becomes inoperative while the output of a second speed sensor remains constant. For a brief period from time t2 to t3, for reasons such as set forth above, an output from the first or primary sensor is produced. This results in the output of the first sensor influencing the actually used speed signal (D) in the manner indicated.